On the Oxidation State of Manganese Ions in Li-Ion Battery Electrolyte Solutions

摘要

We demonstrate herein that Mn~(3+) and not Mn~(2+), as commonly accepted, is the dominant dissolved manganese cation in LiPF_6-based electrolyte solutions of Li-ion batteries with lithium manganate spinel positive and graphite negative electrodes chemistry. The Mn~(3+) fractions in solution, derived from a combined analysis of electron paramagnetic resonance and inductively coupled plasma spectroscopy data, are ~80% for either fully discharged (3.0 V hold) or fully charged (4.2 V hold) cells, and ~60% for galvanostatically cycled cells. These findings agree with the average oxidation state of dissolved Mn ions determined from X-ray absorption near-edge spectroscopy data, as verified through a speciation diagram analysis. We also show that the fractions of Mn~(3+) in the aprotic nonaqueous electrolyte solution are constant over the duration of our experiments and that disproportionation of Mn~(3+) occurs at a very slow rate.